Thanks for the ideas... all very good! I ended up solving the problem my own way.

I will leave my luggage at the hotel in Ollaytantambo, take the train back there after visiting MP, collect luggage and take a taxi back to Cuzco.

This allows me much greater flexibility with the train schedule as there are many more trains to Ollaytantambo than Cuzco. Even with the taxi, it saves some money too as there are three of us traveling.

However, I lose out on the train ride between Ollaytantambo and Cuzco and the switchbacks heading into Cuzco. Is this something not to miss or is the journey in the half closer to MP the highlight?